1. Field of the Invention
The present invention generally relates to a thin film magnetic memory device. More particularly, the present invention relates to a random access memory (RAM) including memory cells having a magnetic tunnel junction (MTJ).
2. Description of the Background Art
An MRAM Magnetic Random Access Memory) device has attracted attention as a memory device capable of non-volatile data storage with low power consumption. The MRAM device is a memory device capable of non-volatile data storage using a plurality of thin film magnetic elements formed in a semiconductor integrated circuit and also capable of random access to each thin film magnetic element.
In particular, recent announcement shows that the use of thin film magnetic elements having a magnetic tunnel junction (MTJ) as memory cells significantly improves performance of the MRAM device. The MRAM device including memory cells having a magnetic tunnel junction is disclosed in technical documents such as “A 10 ns Read and Write Non-Volatile Memory Array Using a Magnetic Tunnel Junction and FET Switch in each Cell”, ISSCC Digest of Technical Papers, TA7.2, February 2000, and “Nonvolatile RAM based on Magnetic Tunnel Junction Elements”, ISSCC Digest of Technical Papers, TA7.3, February 2000.
FIG. 48 is a conceptual diagram illustrating the structure of a memory cell having a magnetic tunnel junction (hereinafter, sometimes simply referred to as “MTJ memory cell”) and the data read operation.
Referring to FIG. 48, the MTJ memory cell includes a tunneling magneto-resistance element TMR having an electric resistance varying according to the storage data level, and an access transistor ATR for forming a path of a sense current flowing through tunneling magneto-resistance element TMR in the data read operation. For example, access transistor ATR is a field effect transistor, and is coupled between tunneling magneto-resistance element TMR and a ground voltage VSS.
Tunneling magneto-resistance element TMR has a ferromagnetic material layer FL having a fixed magnetization direction (hereinafter, sometimes simply referred to as “fixed magnetic layer”), and a ferromagnetic material layer VL that is magnetized in the direction corresponding to an external magnetic field (hereinafter, sometimes simply referred to as “free magnetic layer”). A tunneling barrier TB of an insulator film is interposed between fixed magnetic layer FL and free magnetic layer VL. Free magnetic layer VL is magnetized either in the same (parallel) direction as, or in the opposite (antiparallel) direction to, that of fixed magnetic layer FL according to the storage data level.
For the MTJ memory cell are provided a write word line WWL for data write operation, a read word line RWL for data read operation, and a bit line BL serving as a data line for transmitting an electric signal corresponding to the storage data level in data read and write operations.
In data read operation, access transistor ATR is turned ON in response to activation of read word line RWL. This allows a sense current Is to flow through a current path formed by bit line BL, tunneling magneto-resistance element TMR, access transistor ATR and ground voltage VSS.
The electric resistance value of tunneling magneto-resistance element TMR varies according to the relation between the respective magnetization directions of fixed magnetic layer FL and free magnetic layer VL. More specifically, when fixed magnetic layer FL and free magnetic layer VL have the same (parallel) magnetization direction, tunneling magneto-resistance element TMR has a smaller electric resistance value than when they have opposite (antiparallel) magnetization directions. Hereinafter, the electric resistance values of the tunneling magneto-resistance element corresponding to the storage data levels “1”, “0” are respectively denoted with R1 and R0, where R1>R0.
The electric resistance value of the tunneling magneto-resistance element TMR varies according to the magnetization direction. Accordingly, two magnetization directions of free magnetic layer VL in tunneling magneto-resistance element TMR can be stored as two storage data levels (“1”, “0”), respectively. In other words, free magnetic layer VL corresponds to a storage node of the MTJ memory cell.
A voltage change that occurs at tunneling magneto-resistance element TMR in response to sense current Is varies depending on the magnetization direction of free magnetic layer VL, that is, the storage data level. Therefore, sense current Is is supplied to tunneling magneto-resistance element TMR after precharging bit line BL to a prescribed voltage, and the storage data in the MTJ memory cell can be read by sensing a voltage change on bit line BL.
FIG. 49 is a conceptual diagram illustrating data write operation to the MTJ memory cell.
Referring to FIG. 49, in data write operation, read word line RWL is inactivated and access transistor ATR is turned OFF. In this state, a data write current is supplied to write word line WWL and bit line BL in order to magnetize free magnetic layer VL in the direction corresponding to the write data. The magnetization direction of free magnetic layer VL is determined by combination of the respective directions of the data write current flowing through write word line WWL and bit line BL.
FIG. 50 is a conceptual diagram illustrating the relation between the direction of the data write current and the magnetization direction in data write operation.
Referring to FIG. 50, the abscissa Hx indicates the direction of a data write magnetic field H(BL) generated by the data write current flowing through bit line BL. The ordinate Hy indicates the direction of a data write magnetic field H(WWL) generated by the data write current flowing through write word line WWL.
The magnetization direction of free magnetic layer VL can be rewritten only when the sum of the data write magnetic fields H(BL) and H(WWL) reaches the region outside the asteroid characteristic line shown in the figure. In other words, the magnetization direction of free magnetic layer VL will not change if an applied data write magnetic field corresponds to the region inside the asteroid characteristic line.
In order to rewrite the data stored in tunneling magneto-resistance element TMR by data write operation, a current of at least a prescribed level must be applied to both write word line WWL and bit line BL. Once the magnetization direction, that is, the storage data, is written to tunneling magneto-resistance element TMR, it is held in a non-volatile manner until another data write operation is conducted.
In data read operation as well, sense current Is flows through bit line BL. However, sense current Is is generally about one to two orders smaller than the data write current. Therefore, it is less likely that the storage data in the MTJ memory cell is erroneously rewritten by sense current Is in the data read operation.
With reduction in memory cell size, the MRAM device using such a tunneling magneto-resistance element TMR has the following problems:
The MTJ memory cell stores the data according to the magnetization direction of free magnetic layer VL. Provided that the magnetic layer has a thickness T and a length L in its magnetization direction, the magnetic field strength that must be applied to rewrite the magnetization direction of the free magnetic layer (hereinafter, sometimes referred to as “switching magnetic field strength”) is proportional to T/L. Accordingly, with reduction in memory cell size, the switching magnetic field strength is increased according to the scaling of the size in the in-plane direction.
Moreover, with reduction in memory cell size, magnetic field interference between the fixed magnetic layer and the free magnetic layer is increased inside and outside the MTJ memory cell. As a result, the threshold value of a data write magnetic field required for data write operation (which corresponds to the asteroid characteristic line in FIG. 50) varies depending on the write data pattern or becomes asymmetric depending on the direction of the data write magnetic field.
Such a phenomenon hinders scaling of the MTJ memory cell. Therefore, current consumption is increased with reduction in memory cell size.
In order to solve the above problems, U.S. Pat. No. 6,166,948 discloses the technology of forming a free magnetic layer of an MTJ memory cell from two ferromagnetic material layers having different magnetic moments. Hereinafter, the structure of the free magnetic layer formed from two magnetic layers is sometimes referred to as “two-layer storage node structure”. The structure of the free magnetic layer formed from a single magnetic layer as shown in FIGS. 48, 49 is sometimes referred to as “single-layer storage node structure”.
FIG. 51 is a cross-sectional view of a conventional tunneling magneto-resistance element having a two-layer storage node structure.
Referring to FIG. 51, the conventional tunneling magneto-resistance element includes an antiferromagnetic material layer AFL, a fixed magnetic layer FL, free magnetic layers VL1, VL2, a tunneling barrier TB formed between fixed magnetic layer FL and free magnetic layer VL1, and an intermediate layer IML formed between free magnetic layers VL1, VL2. Intermediate layer IML is formed from a non-magnetic material. The MTJ memory cell having the tunneling magneto-resistance element of FIG. 51 stores the data according to the relation between the respective magnetization directions of fixed magnetic layer FL and free magnetic layer VL1.
Free magnetic layers VL1, VL2 are arranged with intermediate layer IML interposed therebetween. The magnetic moment of free magnetic layer VL1 is greater than that of free magnetic layer VL2. Accordingly, the magnetization threshold value for changing the magnetization direction of free magnetic layer VL1 is larger than that of free magnetic layer VL2.
As described above, free magnetic layers VL1, VL2 have different magnetic moments. Therefore, when the magnetization direction of free magnetic layer VL1 changes, the magnetization direction of free magnetic layer VL2 also changes so that free magnetic layer VL2 forms a magnetization loop together with free magnetic layer VL1.
FIG. 52 is a hysteresis diagram illustrating magnetization in the tunneling magneto-resistance element in FIG. 51. FIG. 52 shows the magnetization behavior in the easy-axis direction of free magnetic layers VL1, VL2 in response to a data write magnetic field H.
Hereinafter, how the magnetization direction changes as the data write magnetic field is increased in the negative direction will be described with reference to FIG. 52.
In the region of H>H01 (state 1A), both free magnetic layers VL1, VL2 are magnetized in the positive direction (to the right). For H<H01 (state 2A), only the magnetization direction of free magnetic layer VL2 having a smaller magnetic moment is inverted.
When the magnetic field is further changed in the negative direction into the region exceeding a threshold value −H02 (state 3A), the magnetization direction of free magnetic layer VL1 having a larger magnetic moment changes from the positive direction to the negative direction (from right to left). Accordingly, the magnetization direction of free magnetic layer VL2 is also inverted from the state 2A.
When the data write magnetic field H is further increased in the negative direction into the region of H<−H03 (state 4A), the magnetization directions of both free magnetic layers VL1, VL2 change to the negative direction (to the left).
Hereinafter, how the magnetization direction changes as the data write magnetic field H is increased in the positive direction will be described.
In the region of H<−H01 (state 4B), both free magnetic layers VL1, VL2 are magnetized in the negative direction (to the left). For H>−H01 (state 3B), only the magnetization direction of free magnetic layer VL2 having a smaller magnetic moment is inverted.
When the magnetic field is further changed in the positive direction into the region exceeding a threshold value H02 (state 2B), the magnetization direction of free magnetic layer VL1 having a larger magnetic moment changes from the negative direction to the positive direction (from left to right). Accordingly, the magnetization direction of free magnetic layer VL2 is also inverted from the state 3B.
When the data write magnetic field H is further increased in the positive direction into the region of H>H03 (state 1B), the magnetization directions of both free magnetic layers VL1, VL2 change to the positive direction (to the right).
The free magnetic layers are formed from antiferromagnetic material layers having different magnetization threshold values (magnetic moments), and a non-magnetic intermediate layer is interposed therebetween. The state where the magnetic fields in the upper and lower free magnetic layers are inverted with respect to each other is used as a data storage state. This enables reduction in switching magnetic field strength of the free magnetic layers. Moreover, in the data storage state, the two free magnetic layers are magnetized in a looped manner. This prevents a magnetic flux from being extended outside the MTJ memory cell, thereby suppressing adverse effects of the magnetic field interference.
In the MTJ memory cell having a two-layer storage node structure of FIG. 51, however, free magnetic layers VL1, VL2 must have different magnetization threshold values (magnetic moments). Accordingly, two magnetic layers of different materials must be deposited with different thicknesses, thereby complicating the manufacturing apparatus and manufacturing process.
In particular, as shown in FIG. 52, the difference between the magnetic moments of free magnetic layers VL1, VL2 significantly affects the data storage state. Therefore, manufacturing variation of the magnetic moments may possibly cause significant variation in data storage characteristics of the MTJ memory cells.
As shown in FIGS. 48, 49 and 52, in the MTJ memory cell, free magnetic layers VL, VL1, VL2 that are magnetized in the direction according to the storage data are formed near fixed magnetic layer FL and antiferromagnetic material layer AFL having a fixed magnetization direction. Therefore, magnetization characteristics in the free magnetic layers may become non-uniform according to the storage data level.
FIG. 53 is a conceptual diagram illustrating non-uniformity of magnetization characteristics in the MTJ memory cell having a single-layer storage node structure.
Referring to FIG. 53, fixed magnetic layer FL and antiferromagnetic material layer AFL have the same fixed magnetization direction. Antiferromagnetic material layer AFL is provided in order to more strongly fix the magnetization direction of fixed magnetic layer FL.
Free magnetic layer VL serving as a storage node is magnetized either in the positive (+) or negative (−) direction according to the storage data level. In FIG. 53, the magnetization direction parallel to that of fixed magnetic layer FL is defined as positive direction, and the magnetization direction antiparallel to that of fixed magnetic layer FL is defined as negative direction.
Since the plurality of magnetic layers are formed dose to each other, a uniform magnetic field ΔHp is applied to free magnetic layer VL in the easy-axis direction due to magnetostatic coupling of the magnetic fields from antiferromagnetic material layer AFL and fixed magnetic layer FL. Uniform magnetic field ΔHp acts in the direction antiparallel to the magnetization direction of fixed magnetic layer FL, that is, in the negative direction. Such a uniform magnetic field ΔHp makes the magnetization characteristics in free magnetic layer VL asymmetric depending on the direction of the magnetic field.
FIG. 54 is a hysteresis diagram illustrating magnetization characteristics in free magnetic layer VL of FIG. 53. FIG. 54 shows magnetization behavior of free magnetic layer VL in response to a data write magnetic field Hex of the easy-axis direction.
Referring to FIG. 54, in order to magnetize negatively magnetized free magnetic layer VL in the positive direction, a magnetic field Hex of the positive direction beyond +Hsp must be applied thereto. On the other hand, in order to magnetize positively magnetized free magnetic layer VL in the negative direction, a magnetic field Hex of the negative direction beyond −Hsn must be applied thereto.
Due to the uniform magnetic field ΔHp resulting from magnetostatic coupling with fixed magnetic layer FL, the magnetization threshold value Hsp of the positive direction is larger than the magnetization threshold value Hsn of the negative direction by ΔHp. Since free magnetic layer VL has asymmetric magnetization characteristics according to the direction of an applied magnetic field, the strength of the magnetic field required to be applied to free magnetic layer VL varies depending on the write data level. In order to use such a tunneling magneto-resistance element as a memory cell, a magnetic field exceeding the larger magnetization threshold value must be applied regardless of the write data level. In other words, a data write current for generating a magnetic field exceeding the magnetization threshold value Hsp must be applied even when free magnetic layer VL is to be magnetized in the negative direction. In this case, an unnecessarily large data write current is required. This may possibly cause increased power consumption and increased current density in the wirings, resulting in degraded wiring reliability.
Such a phenomenon also occurs in a tunneling magneto-resistance element having a two-layer storage node structure.
FIG. 55 is a conceptual diagram illustrating non-uniformity of magnetization characteristics in the MTJ memory cell having a two-layer storage node structure.
Referring to FIG. 55, in a tunneling magneto-resistance element having a two-layer storage node structure as well, a uniform magnetic field ΔHp is applied to free magnetic layer VL1 in the easy-axis direction due to magnetostatic coupling between antiferromagnetic material layer AFL and fixed magnetic layer FL, as in the case of the single-layer storage node structure. Such a uniform magnetic field ΔHp makes the magnetization behavior in free magnetic layers VL1, VL2 in the easy-axis direction asymmetric.
FIG. 56 is a hysteresis diagram illustrating magnetization characteristics in free magnetic layer FL in FIG. 55.
Referring to FIG. 56, the magnetization behavior of free magnetic layers VL1, VL2 in response to a data write magnetic field Hex of the easy-axis direction is shifted by ΔHp with respect to the theoretical characteristics shown in FIG. 52 due to the uniform magnetic field ΔHp produced by magnetostatic coupling with fixed magnetic layer FL. In other words, threshold values −H01′, −H02′, −H03′ for a magnetic field of the negative direction are respectively shifted by ΔHp toward threshold values +H01, +H02, +H03 for a magnetic field of the positive direction shown in FIG. 52. As a result, magnetization characteristics are asymmetric between the magnetic fields of the positive direction and negative directions. In other words, H01−|H01′|=H02−|−H02′|=H03−|−H03′|=ΔHp.
As described above, in both tunneling magneto-resistance elements having a single-layer storage node structure and a two-layer storage node structure, an unnecessarily high data write current level must be applied due to the asymmetric magnetization characteristics.